The present invention is related to fault detection circuits for detecting faults in output stages, and protecting components in the output stages in the event of a fault. More specifically, the present invention is related to solenoid driver output stages and fault detection circuits utilized for detecting faults therein.
In known solenoid fuel injector driver output stages, an output power transistor is typically series connected with a solenoid fuel injector coil and a current sensing resistor. The through current of this series connection is sensed by the resistor and utilized by a current limit controller to provide pulse width modulated output signals that are used as a switching excitation signal for the output transistor. In this manner, an effective minimum current for the solenoid coil can be assured during an on-time duration for the fuel injector which corresponds to the existence of a predetermined logic state of a control signal received by the current limit controller.
In such systems, the current limit controller implements current control between predetermined limits for the through current in the solenoid coil and the output transistor. However, short circuits to ground can occur at the end terminals of the solenoid coil, and this can result in having the sensing resistor mistakenly sense that a low or zero current is flowing through the power output device and solenoid coil. This means that the current controller now does not function to limit the current in the coil and output transistor.
Some prior systems have attempted to detect when a short circuit is present at one of the two end terminals of the solenoid coil, and in response to such a fault detection these prior systems have attempted to turn off the power output transistor to protect it and the solenoid coil from excessive current and voltage stress. However, typically the prior systems have been extremely complex, have not been automatically resettable, have not reacted fast enough to the presence of some short circuits to prevent excessive current from flowing through the power transistor for an appreciable time, and/or have not been able to satisfactorily detect the presence of short circuits at either of the two end terminals of the solenoid coil. Because of this, prior fault detection systems have not adequately protected the output stage in the event of a short circuit, and/or have not done so cost effectively. This same sort of fault detection problem exists not only with solenoid driver output stages, but also can exist with voltage regulator output stages which provide field current for the field coil of a vehicle battery charging system. The problem also can exist in switching power supplies.